Ohmic contacts optimisation for high-power InGaAs-AlAs double-barrier resonant tunnelling diodes based on a Dual-Exposure E-Beam lithography approach

In this paper, we report on a simple test structure which can be used to accurately extract the specific contact resistivity ρc associated with metal-n++ InGaAs-based low-resistance Ohmic contacts through the transfer length method (TLM). The structure was designed to avoid common measurement artifacts that typically affect standard layouts. Moreover, microfabrication was optimised to achieve an accurate short minimum gap spacing of 1 μm through a dual-exposure step based on e-beam lithography, which is required for a reliable ρc estimation. Ohmic contacts based on a Ti/Pd/Au metal stack were fabricated and characterised using the proposed structure, resulting in an extracted ρc ≃ 1.37×10−7 Ω cm2 = 13.7 Ω μm2. This work will assist in increasing the quality of Ohmic contacts in high-power InGaAs/AlAs double-barrier resonant tunnelling diodes (RTDs), and so help to overcome one of the bottlenecks to the output power capability of RTD-based oscillators at terahertz frequencies.